This invention relates to a method of making an electronic device of the type having its electronic element contained inside a casing such that it makes electrical contacts through terminals having an elastic spring-like contact part. In particular, this invention relates to a method of inserting such an electronic element and its terminals into a casing.
One of the examples of such an electronic device that is of interest from the point of view of this invention is a device having a thermistor with a positive temperature coefficient (PTC) as its electronic element contained inside a casing. FIGS. 8A and 8B show an example of prior art thermistor element 1 characterized as having two main surfaces facing outward away from each other and electrodes 2 and 3 formed thereon. Silver used to be the material for forming such electrodes, but silver tends to cause the phenomenon of migration, especially when used under a dewing condition which is likely to cause migration and a short circuit, damaging the thermistor element 1. For this reason, it has been known to form the electrodes 2 and 3, as shown in FIGS. 8A and 8B, by first providing a first electrode layer 4 and then a second electrode layer 5 thereon such that peripheral areas of the first electrode layer 4 remain exposed, the first electrode 4 layer comprising a metallic material such as nickel that can provide an ohmic contact but is not likely to cause a migration and the second electrode layer 5 comprising silver. Since the first and second electrode layers 4 and 5 thus formed remain nearly at the same potential, even if the silver of the second electrode layer 5 is ionized, the electrostatic force on the ionized silver is extremely weak, not significantly contributing to the migration of the silver ions.
FIG. 9 shows a prior art thermistor device 6 obtained by putting a thermistor element 1 as shown in FIGS. 8A and 8B inside a casing 7 made, for example, of a resin, glass, ceramic or metal material. Since the thermistor element 1 is a heat-producing element and reaches a high temperature when in operation, it is commonly made and sold in the form of such a device 6. The device 6 also includes two terminals 8 and 9 made of stainless steel or a copper alloy, each having a contact part 10 contacting a corresponding one of the electrodes 2 and 3 of the thermistor element 1 and an extended part 11 extending from the corresponding contact part 10. The contact parts 10 are each formed so as to serve as a spring, adapted to elastically contact the electrodes 2 and 3.
The casing 7 has throughholes 12 formed therethrough, and the thermistor element 1 and the contact parts 10 of the terminals 8 and 9 are contained inside the casing 7 such that the thermistor element 1 is sandwiched and supported between and by the contact parts 10 of the terminals 8 and 9 which pass through the throughholes 12 through the casing 7, leaving the extended parts 11 extending outside.
According to common methods of producing a thermistor device thus structured, the terminals 8 and 9 are first set at specified positions inside the casing 7 and the thermistor element 1 is then inserted between the contact parts 10 of the two terminals 8 and 9. When the thermistor element 1 is thus inserted between the contact parts 10, the contact parts 10 are deformed against its elastic spring force such that the gap therebetween is increased and that they can admit the thermistor element 1 therebetween while they remain in contact with the thermistor element 1. In other words, the electrodes 2 and 3 are rubbed against the contact parts 10 as the thermistor element 1 is inserted into the casing 7 inside which the terminals 8 and 9 are already installed. This tends to scratch and damage the electrodes 2 and 3, as shown schematically at 13 in FIG. 10.
One of the factors contributing to the scratches on the electrode surface is the shape of the contact parts 10. In order to minimize the heat conduction from the thermistor element 1 to the terminals 8 and 9, the width of the terminals 8 and 9 is reduced at their contact parts 10 as shown in FIG. 11 so as to reduce as much as possible the area through which the contact parts 10 contact the electrodes 2 and 3. These contact areas are much smaller than the surface area of the electrodes 2 and 3 and hence the pressure applied by the contact parts 10 on (or the force on a unit area of) the electrodes 2 and 3 is relatively large, or large enough to scratch the electrodes 2 and 3. Such scratches (as shown at 13) usually affect the quality and/or the function of the electronic element (such as the thermistor element 1). In the case of the thermistor element 1 with a particular structure as described above, the scratches 13 may extend beyond the second electrode layer 5 and may reach the exposed part of the first electrode layer 4 surrounding the second electrode layer 5. This has the undesirable effect of effectively causing the second electrode layer 5 to come closer through the scratch 13 to the main surface of the thermistor element 1. If the thermistor device 6 having such scratches 13 is used under a dewing condition, the silver of the second electrode layer 5 may begin to migrate around the scratch 13.
Worse still, since a relatively large force is applied to the outer periphery of the thermistor element 1 when it is being inserted into the casing 7, the outer periphery of the thermistor element 1 may develop cracks.
It is therefore an object of this invention to provide a method of making an electronic device of the type described above with which the problems as described above can be overcome.
Methods embodying this invention, with which the above and other objects can be accomplished, may be characterized broadly as comprising the steps of providing an electronic element such as a thermistor element with electrodes formed on its two mutually parallel outwardly facing main surfaces, at least two terminals and a casing as described above, and inserting the electronic element and the terminals into the casing either simultaneously or sequentially such that the electrodes are not rubbed by the contact part of either of the terminals. There are many ways to prevent such rubbing of the electrodes by the contact parts of the terminals. One of the ways is to insert the terminals first and then cover their contact parts with flexible planar guide plates such that the electrodes will glide over them as the electronic element is inserted. Another method is to sandwich the electronic element between the terminals with the contact parts of the latter contacting the electrodes on the main surfaces of the former to thereby preliminarily form an assembly consisting of the electronic element and the contacting terminals and then to insert this assembly into the casing. For this purpose, the casing may be made divisible into halves such that the pre-formed assembly is inserted into one of them and then the two halves are joined together. Alternatively, the casing may be made divisible into a tubular main body with one open surface and a lid with throughholes for closing this open surface. The terminals are preliminarily passed through these throughholes and supported by the lid. With one or both of these throughholes appropriated configured, one or both of the terminals passing therethrough can be tilted such that they can be opened as the electronic element is inserted and then closed to form an assembly which is then inserted as a whole into the main body. These methods are also applicable for making an electronic device containing two or more electronic elements inside a casing. The electrodes on the electronic element may be of a layered structure with the inner layer made of a metal less likely to general migration than the metal material of the outer layer.